1. Field of Invention
The invention generally relates to optical cross connects. More specifically, the invention relates to signaling architectures and methods in an optical cross connect.
2. Description of Related Art
Switches are provided in communication networks in order to direct traffic to a desired destination. As the use of networks has increased over time, so has the need for more bandwidth. Fiber optic networks were developed to meet this need and transmit data (e.g., voice and data signals) at high data rates. The American-based Synchronous Optical Network (“SONET”) standard and the corresponding European equivalent standard, Synchronous Data Handling (“SDH”), are examples of two industry standards developed for the transmission of data over such fiber optic mediums. For simplicity the remaining description of optical-based networks will focus upon the SONET standard. However, those skilled in the art will recognize that the concepts as they pertain to SONET are also applicable to SDH and other data transmission protocols.
In a SONET-based network data is transmitted as a series of multiplexed time slots or frames. The lowest data rate transmission typically within a SONET network is at a base rate of 51.84 Mbit/second, which in the electrical domain is referred to as a synchronous transport signal—level one (“STS-1”) frame, and in the optical domain corresponds to an Optical Carrier—level 1 (“OC-1”) frame.
Higher data rate frames can be formed from integer multiples of STS-1s, and are designated as STS-N/OC-N with N being a value, such as 3, 12, 48, 192, etc. For example, an OC-3 transmission is three times the base rate of OC-1.
As seen in FIG. 1, an OC-48 signal, when converted to corresponding electrical signals, includes 48 STS-1 frames. Each STS-1frame is transmitted during a respective time slot, and comprises two components: a transport overhead and a payload. The transport overhead is provided in 9 rows of three bytes each (27 bytes total), and carries administrative information used by network elements to manage the transfer of the frame through the network. The payload, referred to as the Synchronous Payload Envelope (“SPE”), is provided in 9 rows of 87 bytes each (783 bytes total) and comprises the major portion of an STS-1. The SPE carries payload and STS Path Overhead (“STS POH”) bytes, and may begin at any byte location within the payload envelope, as indicated by a pointer value in the overhead.
As mentioned above, each STS-1 segment includes a payload section and an overhead section. The overhead includes K-bytes that communicate error conditions between spans in a network and allow for link recovery after network failure. K-byte signaling takes place over the protection lines. In a series of STS segments, only K-bytes from the first STS-1 segment are used to carry error data. Current SONET networks make no use of the framing overhead of the remaining STS-1 segments. The series of STS-1 segments only carries K-byte error information for a single ring.
The B1 byte is a single interleaved byte used to provide error monitoring on the previous STS-n. B1 is also known as BIP-8 and stands for bit-interleaved parity. It performs even-parity checking on the previous STS-n frame in the SONET stream. This byte is only defined in the first STS-1 within an STS-n. B1 is a Section overhead byte.
The B3 is a Path overhead byte. It is analogous to the B3 (i.e. BIP-8) but is calculated over the previous SPE prior to scrambling.
SONET networks often have a ring configuration including a collection of nodes forming a closed loop. FIG. 2 illustrates an example of a conventional SONET bidirectional ring 100 whereby information may flow in either a clockwise or counterclockwise in the figure, as indicated by arrows labeled “working” and “protect”. Add-drop multiplexers (A/D mux) 110, 120, 130 and 140 add and/or drop signals to switch data from one span (SP1 to SP7) to another. Ring 100 is thus termed a “bidirectional line switched ring” or BLSR, and data transmitted in such a ring typically must conform to a particular protocol.
As further shown in FIG. 2, each of spans SP1 to SP7 includes one working line and a corresponding protection line. For example, spans SP1 and SP5 interconnect A/D muxes 110 and 120 and include working lines carrying data in opposite directions. The working lines within each of these spans further include respective protection lines for transmitting data in the event the associated working line fails.
The SONET ring provides protection for transmission of data in two ways. First if a working lines fails, the corresponding protection lines may be used. In the alternative, if working lines fail between two A/D muxes, any communication route directed through the failed line may be rerouted through the A/D muxes through a process known as span switching. For example, if the working lines between A/D mux 110 and A/D mux 120 fail, instead of using the corresponding protection lines, communications may be sent from A/D mux 110 to A/D mux 120 via A/D mux 140 and 130.
Typically, the working and protect lines are provided in a fiber optic bundle. Accordingly, if the working line fails, due to a fiber cut, for example, the corresponding protect line often will also fail. Span switching is thus often preferred to simply switching data from the faulty working line to the protect line (also known as “1+1 protection”). Both schemes may be used in conjunction with each other, however, whereby an attempt is first made to use the protect line when the associated working line fails, and then, if the protection line is itself faulty, span switching is used to redirect communications.
To provide protection switching, SONET equipment must be able to monitor incoming signals and make decisions based on the quality of the signal. The quality of the signal is generally derived from standard SONET error conditions. These can include LOS (Loss Of Signal), LOF (Loss of Frame), LOP (Loss of Pointer), K1, K2, B1, and/ or B3 bytes etc. K1, K2 bytes are specifically defined as APS (Automatic Protection Switching) bytes and provide this function at the Line and Section level. B1, B3 are parity indicators to detect signal deterioration.
When the first SONET rates (OC-1,OC3 etc) where first implemented, protection schemes where implemented at the Line and Section level. The Path layer held the same amount of information as the Line and Section layer. Basically the envelope was just as big as the contents. Now that SONET technology has grown to must faster rates (OC-48, OC-192), the envelop has many packages in them. Since it is rare that one customer uses a complete OC-192 or OC-48, these rates are generally used as a result of aggregation of the slower rates. Thus, carriers are now faced with large SONET pipes and various protection requirements for each of the sub STS-n frames incorporated in those pipes.
These new requirements from carriers have now spawned the development of cross connects that support large bandwidths but can yet protect down to the STS-1 level. One problem generated by such requirements is that conventional architectures used for automatic protection switching can no longer be implemented that satisfy these requirements. This is due to the fact that these conventional architectures are generally software-intensive and thus increasing the capacity by two orders of magnitude would require the same increase in processing power. This would increase the cost of microprocessors running the APS software considerably and thereby make the systems too expensive and unmarketable.
The invention described herein provides a unique cost-effective solution to these problems.